The present invention relates to flame retardation of polyamides with particular regard to non-halogen flame retardants.
Flame retardation of thermoplastic polyamides by addition of various relatively low molecular weight (i.e. non-polymeric) organic additives, frequently together with antimony oxide, is known in the art. Typical organic additives include polybrominated aromatic hydrocarbons and ethers, as well as polychlorinated polycyclic hydrocarbons. These additives are not, however, wholly satisfactory because they generate smoke and corrosive hydrogen halides upon burning, and in some cases are subject to toxicological and environmental concerns. They also require sufficiently high loading so as to cause significant loss of some of the desirable physical properties of the polyamide.
Phosphorus compounds have also been used to flame retard thermoplastic polyamides. However, they are relatively inefficient and tend, in many cases, to cause undesired plasticization as well as degradation during processing. Red phosphorus has been used, but poses some handling hazard and can cause evolution of some phosphine during processing or thereafter.
Polyamides processed at lower temperatures can be flame retarded with ammonium polyphosphate or organic phosphates, but nylons processed at higher temperatures, such as above 200.degree. C. are damaged by such additives, probably due to acid formation. It has been especially difficult to flame retard nylon 4,6 which is processed above 300.degree. C., since only a few aromatic halogen additives (which entail the problem mentioned above) have enough stability. Furthermore, it has not been possible to find an available phosphorus-based flame retardant additive with enough stability or inertness towards the polyamide.
One object of the present invention is to provide a non-plasticizing, non-corrosive and non-degrading flame retardant additive for thermoplastic polyamides. Such additives are provided particularly for polyamides processed above 200.degree. C. up to the temperature of unacceptable decomposition of the polymer itself, which for the most stable polyamides, may be as high as 400.degree.-500.degree. C. The present invention is especially useful for flame retarding nylon 4,6 which is processed at above 300.degree. C.